The use of thin gauge rigid plates or sheets in automobile manufacture has been greatly accelerated due to improved corrosion protection, lighter weight, easy formability and lower cost. This practice has left certain parts of a car more vulnerable to easy denting and damage. Historically, either a reinforcing plate is welded or bonded to these areas, or a precut patch of epoxy impregnated glass fibers is attached.
Reinforcing compositions prepared from thermosetting resins, particularly epoxy resins, used to reinforce thin rigid sheets used in automobiles exhibit a number of advantages including strength, adhesion, and corrosion resistance. One distinct disadvantage is that many thermosetting resins, particularly epoxy based compositions, deform the substrates to which they bond due to shrinkage during curing.
In the automobile industry this deformation is of particular concern where the appearance of the substrate is critical. This substrate deformation necessitates additional costs in labor and materials in order to restore the intended appearance of the substrate.
U.S. Pat. No. 5,151,327 discloses an adhesive sheet for reinforcing thin rigid plates. Such adhesive sheets comprise several resinous layers and need to undergo a sizing treatment before application to the thin rigid plates. The adhesive sheets contain foaming agents to prevent shrinkage during cure. Application is by lamination.
U.S. Pat. No. 4,739,019 discloses an adhesive composition for use in automotive applications. This composition contains an epoxide and a thermoplastic polyester. Such a composition loses its strength when heated to curing temperatures because the viscosity of the thermoplastic polyester decreases, and when applied to the underside of a substrate, the composition tends to sag or fall off the substrate during heating.
U.S. Pat. No. 5,155,138 discloses the use of expandable thermoplastic microspheres in polymers and paints. It is also known to use expandable microspheres in car underbody coatings and sealants. (See Nobel Industries Technical Bulletin no. 21). There is no teaching of the use of such microspheres in thin panel reinforcing compositions.
It is desirable to develop a curable thermosetting composition, preferably sprayable for convenient application, which when used to reinforce thin rigid plates used in automobile manufacture exhibits excellent physical properties and reduced shrinkage.